As the first images of comet Borrelly arrived from the spacecraft, they triggered a raucous ovation in the control room at Cal Tech's Jet Propulsion Laboratory, according to mission manager Marc Rayman.

"To be quite honest, I didn't believe that this little spacecraft was up to the complex and risky assignment that we gave it," Dr. Rayman says. Once the data started coming, he adds, "I was concerned about the seismic risk posed to southern California from the thunderous applause and cheering" in the control room.

Indeed, the researchers note that Deep Space 1 returned the most detailed images yet of a comet's nucleus. Borrelly's oblong core, some 4 kilometers by 8 kilometers, looks like a cosmic bowling pin.

While the object's general shape and reflectivity matched estimates derived from telescope observations from Earth, "what is a surprise is the strong, complex topography of the nucleus," says Laurence Soderblom, a planetary scientist with the US Geological Survey's office in Phoenix and the leader of the mission's imaging team.

"The surface is covered by lots of bright and dark spots," he says, noting that bright and dark are somewhat relative terms. "You have dark soot, and dark, dark, dark soot" on the surface, giving the object the appearance of what one team member called "a Dove Bar the size of Mt. Everest."

First-cut studies of the images show a jumbled surface with elevated mesas and regions from which the comet appears to be spewing the material that forms its coma. Researchers were surprised to find that a jet of dust and gas blasting outward on the sun-facing side - a feature seen on other comets - actually consisted of three parallel jets when seen up close.

Another surprise came from measurements of the comet's interaction with the solar wind - a persistent stream of charged particles hurtling outward from the sun. The nucleus and the flow of ions that form a component of the comet's tail were out of alignment compared with observations of similar features on other comets.

In the coming weeks, the science team will be looking at what the data say about the comet's chemical composition. And by combining images from Deep Space 1's approach to the comet as well as the flyby, the team hopes to build the first 3-D model of a comet's nucleus.

Scientists are eager to comb the data because comets are thought to be construction debris from the formation of the solar system some 4.6 billion years ago. By studying comets, planetary scientists hope to gain insights into conditions that existed when the solar system formed. In addition, comets are thought to be the vehicles that delivered to the young Earth water and other compounds vital to the emergence of organic life.

Launched on Oct. 24, 1998, Deep Space 1 is a munchkin by interplanetary standards. Only about 8 feet tall, the craft tipped the scales at half a ton fully fueled. It was designed to test a dozen cutting-edge technologies for future missions, not to withstand the rigors of hurtling through a comet's coma at 36,900 miles an hour.

In planning the mission, however, managers listed the comet as a potential "bonus science" target.

Rayman adds that in September 1999, as Deep Space 1's primary mission ended with an asteroid flyby, a camera critical to navigation failed. Under other circumstances, the failure would have prompted NASA to pull the plug on the mission, he says. But engineers dragooned another onboard camera - one of the craft's two prototype science packages - into service for navigation, allowing the extended run.

Dr. Rayman says that with Deep Space 1's extended mission over, he'd like to press NASA for a hyper-extended mission. The goal, he says, would be to put the craft's unique ion-drive motor to a new set of engineering tests. The original benchmark declared the motor successful after 200 hours of operation. As of the comet flyby, the motor has operated for more than 14,000 hours.